1. Field of the Invention
The present invention relates to a method for reworking of the residue (bottom product) of extractive distilling processes for obtaining pure hydrocarbons and to an apparatus suitable for performing the method.
2. Brief Description of the Background of the Invention Including Prior Art
Extractive distilling is presently a widely employed method for separating mixtures of materials and in particular for separating mixtures of hydrocarbons, which based on the boiling points of their components cannot be separated or only to an unsatisfactory degree by distilling. As an example, the obtaining of pure aromatic hydrocarbons (benzene, toluene, xylene) from aromatic hydrocarbon containing starting materials, the separation of mono and diolefins as well as the separation of these olefins from non-aromatic hydrocarbons are mentioned. Such extractive distillation is taught for example in U.S. Pat. No. 3,679,579 (to G. Preusser et al.).
Numerous selective solvents have been proposed for performing such extractive distilling processes and N-substituted morpholine, in particular N-formyl-morpholine have proven to be advantageous solvents. The separating effect of the employed selective solvent is based on the effect that in its presence the partial pressures of the individual components of the mixture to be separated are changed such that the vapor pressure differences between those components, which are to be enriched in the extract and those components which are to be enriched in the raffinate is increased. Therefor, the latter can be distilled off as easier boiling fraction from the head of the extractive distilling column, while the less easily boiling components together with the major part of the solvent provide the residue of the extractive distilling column, the so-called bottom product or extract phase.
In order to separate the components to be obtained as an extract from the solvent, therefor the initially described reworking of the residues of the extractive distilling column is required in a following distilling column. Usually the distilling column is operated with a reflux of the components leaving at the head of the distilling column for achieving a good separation effect. The solvent leaving at the bottoms of the distilling column has an elevated temperature, which is for example in the case of the production of aromatic compounds depending on the operating conditions of the distilling column such as pressure and temperature in a region of from about 150.degree. to 240.degree. C. Before feeding back the solvents flowing out of the bottoms of the distilling column into the extractive distilling column it is required to cool the solvent down to from about 90.degree. to 120.degree. C.
In order to employ the heat content of the solvent flowing from the bottom of the distilling column it has been proposed to cool the solvent in indirect heat exchange with other product streams before feeding it back to the extractive distilling column. For example in the advertising flyer Koppers Bericht 333b of September 1969 there is indicated a process scheme for the production of pure o-xylene from reformate by extractive distilling, where the solvent flowing from the distilling column is used first in an indirect heat exchange to heating up the residue product of the distilling column and of the extractive distilling column. This mode of operation, however, does not provide for cooling of the solvent to the extent that its feeding back into the extractive distilling column is possible without further steps. Therefor, it has to be provided with a further cooling in an aircooler before being fed back to the extractive distilling column, whereby of course part of its heat content is lost without providing any use.
In fact, the economy of the solvent recovery from the residue product of the extractive distilling column and the therewith connected use of the heat content of the solvent separated from the hydrocarbons of the extract represent factors, which decisively influence the economy of the total process.